Advanced RFID technology based on integrated circuits (IC) provides very sophisticated tags with non-volatile rewritable memory and anti-collision protocols. The cost of tags is low due to their high manufacturing volumes and the RFID technology is widely adapted to applications where identification is required.
Despite of the several advances of the RFID technology, it still suffers from a few limitations, such as a possibility to precisely locate a tag spatially. So far, a few solutions are proposed for solving this problem. First, a tag could be spatially localized by using several, spatially distributed antennas with phase-shifters in the reader device and scanning a volume with the antenna beam or spot. The spatial resolution of the antenna array depends on its electrical size, and is typically low at relatively low UHF-frequencies. At those frequencies, high resolution would necessitate impractically large antenna arrays to the reader.
One tag ranging technique is based on amplitude measurement of backscattered signal. In this technique, the reader device uses the received signal strength indicator (RSSI) to calculate the distance of the tag. This method, however, assumes that the tag properties are known in terms of its modulation index, antenna gain, and antenna impedance match. As a consequence, several unknown effects, such as tag antenna impedance mismatch due to proximity of metal or dielectric material or tag position, cause a large error to the measured distance of the tag.
The third proposed technique for ranging RFID tags is based on exploitation of the reader device as a stepped-frequency CW radar [D. Arnitz, K. Witrisal, and U. Muehlmann, “Multifrequency continuous-wave radar approach to ranging in passive UHF RFID,” IEEE Transactions on Microwave Theory and Techniques, Vol. 57, No. 5, pp. 1398-1405, May 2009]. In this method, the tags response is measured at several different frequencies and its distance is solved taking the inverse Fourier-transform of the measured frequency response. A drawback of the method is that it requires the frequency response of the tags modulated reflection coefficient. Therefore, the method can only be used with previously characterized tags. Also US 2009/0309780 discloses a method for distance measurement and data transmission in a continuous wave radar system, which suffers from the same disadvantage.